UserNameItem.php

<?php

namespace Drupal\user;

use Drupal\Core\Field\FieldDefinitionInterface;
use Drupal\Core\Field\Plugin\Field\FieldType\StringItem;

/**
 * Defines a custom field item class for the 'name' user entity field.
 */
class UserNameItem extends StringItem {
    
    /**
     * {@inheritdoc}
     */
    public function isEmpty() {
        $value = $this->get('value')
            ->getValue();
        // Take into account that the name of the anonymous user is an empty string.
        if ($this->getEntity()
            ->isAnonymous()) {
            return $value === NULL;
        }
        return $value === NULL || $value === '';
    }
    
    /**
     * {@inheritdoc}
     */
    public static function generateSampleValue(FieldDefinitionInterface $field_definition) {
        $values = parent::generateSampleValue($field_definition);
        // User names larger than 60 characters won't pass validation.
        $values['value'] = substr($values['value'], 0, UserInterface::USERNAME_MAX_LENGTH);
        return $values;
    }

}